The invention concerns a method for the quantized speed control of a stepper motor as well as an arrangement for the implementation of said method.
The method and the arrangement in accordance with the invention are to be used in particular for the speed control of stepper motors for driving the print heads of metal paper printers.
In U.S. Pat. No. 4,101,006, a control circuit is described for a stepper motor driven carriage of a printer, whereby the advance pulse sequence corresponding to a predetermined speed profile can be fetched from a read-only storage.
In U.S. Pat. No. 3,628,119, a method of controlling a stepper motor is described whereby, after the motor has been started by a starting pulse, the motor control is advanced by the pulses from a coded disk, fixed to the motor shaft, and generated during each step, a single additional pulse being applied to the motor control between two step pulses to accelerate the motor from a lower to a higher speed, and one step pulse being suppressed to decelerate the motor from the higher to the lower speed.
Furthermore, U.S. Pat. No. 4,072,888 concerns a method of controlling a stepper motor whereby, after the motor has been started by means of a starting motor advance pulse, feedback pulses dependent upon the motor position are used to control the motor. This method is characterized in that the motor advance pulses triggered by preceding feedback pulses are emitted after a particular delay time has elapsed, whereby said delay time corresponds to the angular magnet wheel value associated with the respective speed or number of steps obtained as well as with the optimum torque.
From IBM Technical Disclosure Bulletin, Vol. 21, No. 4, September 1978, page 1504, it is known to use a coded disk for the position control of a metal paper printer. At its circumference this coded disk is provided with a plurality of equidistant marks which are photoelectrically sensed during rotation (of the stepper motor connected to said disk), in order to generate print pulses. In this manner the print positions along a line are synchronized independently of the motor speed.
For print a line, for example, the stepper motor driven print head of a metal paper printer has a particular speed profile. This speed profile (see FIG. 5) is marked by a starting phase (In1) at the beginning of the print line, a uniform speed phase (In2) during which the speed is approximately uniform, and by a deceleration phase (In3) at the end of the print line.
To optimize the acceleration or deceleration characteristics of the motor, deceleration pulses were applied to the stepper motor to ensure favorable deceleration characteristics, for example. These deceleration pulses acted as delayed standard control pulses. The delay time determined the size of the so-called deceleration angle which will be referred to in detail in connection with FIG. 2. As the time delay was invariably of the same magnitude, the following disadvantage was encountered:
The occurrence of speed fluctuations of the stepper motor (e.g., as a result of load variations and temperature influences) could lead to different deceleration phase angles in the delay period for deriving the deceleration pulse. As a result, the deceleration conditions obtained were not uniform. In practice that meant that for repeated line print processes the end point Z of the speed profile (see FIG. 5) would be displaced along the horizontal axis. A displacement of that point, caused by unfavorable deceleration characteristics leading to motor oscillations, meant that the original line starting point would not be adhered to after print head return and emission of a particular number of position pulses determining the line length.
This disadvantage has little or no effect on metal paper printers with relatively low speeds. However, at increasing print speeds and print resolutions it is no longer tolerable to derive the acceleration pulse from the standard control pulse by means of an electronic time delay circuit.
Therefore, it is the object of the invention to provide means for generating a deceleration or acceleration pulse at particular angular positions (meaning the angular positions of the rotor relative to the stator) of the stepper motor. In addition, it is the object of the invention to provide means for a quantized speed control of a stepper motor.
The objects are attained by providing a coding disk that moves in synchronism with the stepper motor and carries sensible first and second pluralities of marks equiangularly spaced about the disk which define relative displacements of rotor and stator. The first marks define standard control pulse locations, while the second marks further subdivide the distance between successive first marks to provide selectable values of displacement. The selected second marks are used to provide the desired delay in the application of new motor advance pulses. Circuit means including counter means are used to control the selection of actuating secondary marks from which advance pulses are generated. This arrangement provides a delay in the motor advance pulses that are sensitive only to the displacement of the motor.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing.